1. Field of the Invention
This invention relates to traffic interchanges and, more particularly, to a two-level interchange for improving the traffic flow through an intersection of surface streets. The structure directs left-turning traffic onto a relatively small center-turning overpass so as not to interfere with through-traffic.
2. Description of Related Art
Vehicular traffic on many city streets has increased to the point that a single level interchange cannot effectively handle the volume. A conventional 4-way intersection, such as the 3-lane intersection schematically depicted in FIG. 8, has two eastbound lanes 1 and 2, two westbound lanes 3 and 4, two southbound lanes 5 and 6, two northbound lanes 7 and 8, and for each direction one of the two lanes carries left-turning traffic, as indicated by the curved arrows in lanes 2, 4, 6 and 8. Thus, if the right turn paths, from eastbound lane 1 into southbound lane 3, for example, are ignored, the intersection has eight paths through it, any single one of which conflicts with five other paths, while the remaining two paths are mutually conflicting so traffic paths can proceed simultaneously on only two paths. Accordingly, the traffic light at the intersection must cycle through four phases in order to provide an orderly flow of traffic: left-turning cross-traffic; cross traffic, left-turning through traffic and, finally, through traffic. Typically, the traffic light is controlled by a four-phase timing clock set for 15 seconds left-turn; 60 seconds East/West through traffic; 15 seconds left-turn; and 50 seconds North/South through-traffic (15:60:15:50). Since for every phase the red lights are on longer than the green, the intersection will, in most instances, jam in heavy traffic flow.
Multi-level street crossing interchanges which permit continuous traffic flow while common on interstate highways, are impractical for use in a crowded city environment. They usually include large radius cloverleaf turns covering a large land area, and typically also include traffic lanes stacked up on three or more grade levels. The cost of land acquisition to build such an interchange for a busy street intersection would be prohibitive. Moreover, for aesthetic reasons the intersection should not have more than two grade levels.
An example of a typical large interstate intersection is illustrated in U.S. Pat. No. 3,107,590 issued to A. O. Cedeno on Oct. 22, 1963. This intersection requires use of extensive adjoining property to accommodate its wide turning lanes.
U.S. Pat. No. 4,630,961 issued to H. Hellwig on Dec. 23, 1986, extends the teaching of this Cedeno patent, illustrating several attempts to compress Cedeno's design into urban situations. However, to do so required several city blocks and traversing six faces of two city blocks, turning 270 degrees right in order to turn 90 degrees left. If the route is not elevated, it fully corrals the properties of those two city blocks.
U.S. Pat. No. 2,946,267 issued to A. O. Cedeno on Jul. 26, 1960, is illustrative of a traffic intersection which can be used for city streets. However, more than two grade levels are required, all through lines must change in grade level at the street intersection, and right turns are unnatural because they must be initiated from the left side of the street.
A. O. Cedeno's U.S. Pat No. 2,949,067 dated Aug. 16, 1960 discloses a traffic intersection similar to that of U.S. Pat. No. 2,946,267, but provides for left turns from the left side of the street. However, through traffic in the intersection is reversed for the normal flow of traffic.
U.S. Pat. No. 5,049,000 issued to F. D. Mier et al on Sep. 17, 1991, discloses a continuous flow intersection of first and second streets having intersecting through lanes at two grade levels, one at ground elevation and the other either above or below ground. Right turn lanes are all located at grade level, and left turn lanes may originate from the left or right side of each street and cross over to eventually be on the left side of advancing traffic prior to reaching the intersection. Each left turn lane then curves through the intersection to enable traffic to merge with intersecting traffic from a right lane.
Continuous flow intersections are expensive and lend themselves to high speed applications; accordingly, they can only be justified in expensive Federal highway projects. They do not serve local traffic needs.